Heat exchange surface

ABSTRACT

A heating surface element with a zig zag arrangement of longitudinal undulations which serve to minimize the skew flow of the gas. The angle of the undulations is preferably 15° to 35° to the primary flow direction of the gas. When used in combination with a notched heat surface element, the undulation preferably crosses no more than one opposing notch before changing direction.

This invention relates to rotary regenerative air preheaters for boilerplant and industrial furnaces, and more particularly to air preheatersof the kind in which an open-ended cylindrical drum houses amultiplicity of spaced heating-surface elements in the form of undulatedmetal plates, and is rotatable slowly to move the elements into,successively a stream of hot flue gases and a stream of combustion air,said streams flowing axially through the drum and between the elementsin the drum.

According to the prior art, many of the heating-surface elements forsuch air preheaters have typically been of the forms shown in FIG. 1 to10 of the accompanying figures.

FIG. 1 is a fragmentary end view (extracted from GB992413) of packedundulated heating-surface elements and FIG. 2 is a fragmentaryperspective view of the pack of FIG. 1.

Referring to the drawing, typical heating surface elements for an airpreheater of the aforesaid kind consist of undulated metal plates 1, theundulations 2 extending at an angle (θ) of typically 30° to the axis ofthe drum and each alternate undulated plate 1 having axially extendingnotches 3 and 4 formed in its opposite faces to maintain the spacing ofthe plates.

FIG. 3 is a perspective view of a variation of the form of elementsshown in FIG. 2. Close inspection of the view will indicate that theaxially extending notches are closer together than those shown on FIG.2, thereby reducing the number of undulations that are between twoadjacent axial notches. Note that this figure has also been marked withboth the axial flow direction at the inlet to the pack and the smallangle (α₁) to the to the flow direction that can develop when flowpasses through such packs. This slight ‘skew flow’ is caused by theangled direction of the undulations in the sheets—particularly those inthe undulated only sheet. The valleys in these undulations rununderneath the axial notches in the opposing notched sheet and provide aslightly open flow path through which a proportion of the flow candissipate.

FIG. 4 is a perspective view of another variation of the form ofelements shown in FIG. 3. Close inspection of the view will indicatethat the direction of the undulations in the lower sheet without theaxial notches has been reversed, thereby ensuring that the undulationsin adjacent element sheets go in different directions. The elementsshown in FIG. 4 are termed as being “crossed”, while those shown inFIGS. 2 and 3 are termed as being “uncrossed” elements.

Neither the particular reasons for using “crossed” as opposed to“uncrossed” elements nor the performance effects of varying the notchpitch, undulation heights or undulation angle are the subject of thisinvention and will not be discussed further herein. Nevertheless, thedetailed, ‘fine tuning’ of the combination of these factors play animportant part in optimising both the thermal performance and pressuredrop characteristics of these elements. In this case, FIG. 4 shows anembodiment of such designs of elements that illustrate an optimisedversion of this type of profile.

FIG. 4 has also been marked with both the axial flow direction at theinlet to the pack and the slight skew angle (α₂) to the flow directionthat can still develop when flow passes through such a pack—despite the‘crossed’ undulations. Again, this slight skew flow is caused by theangled direction of the undulations in the sheets—particularly those inthe undulated only sheet. This figure has also been marked with theundulation angle (θ), which is typically in the range 15°-35° to theflow direction and is most commonly at 30° to the flow direction. Notefurther that the skew flow angle is generally very much smaller than theundulation angle (α₂<<θ and indeed, for the element profile shown. α₂ istypically only around 20% of θ (i.e. around 6° to the flow direction).

All of the profiles shown in FIGS. 1 to 4 are what are commonly referredto as being double undulated (or DU type) profiles where both sheets inthe element pair are undulated.

By comparison, FIG. 5 is a fragmentary end view of another common formof element profile and FIG. 6 is a fragmentary perspective view of thepack of FIG. 5. This form of element is commonly referred to as being acorrugated undulated (or CU type) profile. In this case, while one sheetin each element pair is a simple undulated sheet similar to those shownin FIGS. 1 to 4, the second sheet has a much deeper series ofcorrugations running axially along the length of the element in the flowdirection. Note that, as with the DU-family of elements, this range-ofCU-type elements also suffers from a tendency to develop a slight skewflow as flow progresses through the element pack.

The detrimental effect of the of the ‘skew’ flow component described onthe performance or operating characteristics of different element packscan be very variable, ranging between having negligible effect to havinga dramatic effect, depending on the detailed geometry of the particularelement. However, the general detrimental effects variously attributedto ‘skew’ flow have been

-   -   1. a possible reduction in thermal performance,    -   2. the production of temperature distributions across the outlet        to the pack. At the cold end of the air heater, this can produce        a “cold corner”, which has been claimed to aggravate cold end        fouling.    -   3. the claimed dissipation of the velocity of the high energy        cleaning jets of steam or compressed air that are conventionally        used to clean these elements    -   4. the tendency for cleaned particles to be transported towards        one side of the element packs during water washing sequences.

Although developed for another reason, FIG. 7 extracted from U.S. Pat.No. 6,019,160 illustrates the main features of another family of heattransfer elements that are commonly referred to as “double notched” orDN-type elements. These elements are named such because both heattransfer elements in every pair contains both axial notches and angledundulations. While this element arrangement does not eliminate thelateral dissipation of flow under the notches, in this case, thearrangement of double notches, combined with the use of a “crossedundulation” format tends to allow the flow to dissipate in either of twodirections when flowing through the pack. Thereby, it has been claimedthat this element profile produces less skew flow effect and lowertemperature stratification at the pack outlet.

In the extreme, skew flow is eliminated in elements which have linecontact between the notched or corrugated sheets and an adjacent flatsurface as shown in the simplest of elements (the low performancenotched-flat profile) on FIG. 8 and in the more complicated higherperformance element as shown in FIG. 9.

Moreover, there have been various attempts made to optimise the balanceof heat transfer performance of these double-notched elements astypified by the element profile shown on FIG. 10 from U.S. Pat. No.6,179,276. Note that this patent involves the use of transverseundulations arranged in a zig-zag pattern.

FIG. 16 depicts a pack of heating surface elements according to U.S.Pat. No. 2,596,642 in which a plurality of ridges are positionedopposite the apexes in a herringbone structure heating surface element.However, this aids the generation of unwanted vortices and furthermoreis difficult to manufacture due to the accurate positioning of theridges needed.

It is thus an aim of the present invention to minimize or at leastreduce the net skew flow of the passing gas.

According to the invention there is provided a heating surface elementhaving first and second adjacent regions, the regions extending alongside each other in a first direction such that the boundary between saidfirst and second regions is in the first direction, each region having aplurality of undulations arranged laterally side by side, eachundulation having a longitudinal extent, the longitudinal extent ofundulations in said first region being arranged between 0° and +90° tosaid first direction and the longitudinal extend of undulations in saidsecond region being arranged between 0° and −90° to said firstdirection.

The skew flow therefore does not all flow in one direction and the netskew flow will be reduced. The first direction is generally the primaryflow and direction of the undulations arranged such that the effect ofthe undulations in the first and second regions is equal and opposite.

It will be appreciated that there may be a space between the two regionsfree from undulations. However, the two regions preferably border eachother directly to ensure that the skew flow is over the greatestpossible area. If the two regions border one another the maximum of thepeaks of the undulations in the first region preferably substantiallymeet the maximum of the peaks of the undulations in the second region.

The invention described above comprises simply a first and secondregion. However, it will be appreciated that there may be a plurality offirst regions and/or a plurality of second regions, with each firstregion alternating with each second region. This prevents the skew flowfrom developing to be too strong. If the first and second regionsdirectly border one another the skew air flow will be directed towards aboundary region before being deflected back towards the oppositeboundary region. The air therefore follows a zig zag pattern along theheating surface element thereby increasing the time taken to pass overthe heating surface element and improving the heat exchange properties.The angle of the longitudinal undulations in the first and second regionwith respect to the first direction are preferably equal and opposite.The angles are preferably between 10° to 40° (and −10° to −40°) and morepreferably 25° to 35° and (−25° to −35°).

A pack, such as a stack, of heating surface elements may comprise aheating surface element according to the invention. The pack of heatingsurface elements may additionally comprise a notched heating surfaceextending in the first direction. The notch can be in the form of asingle, larger undulation or simply a protrusion from the main body ofthe heating surface element which operates to keep the heating surfaceelements apart. The notched heating surface element is preferablyarranged such that each of said regions is directly opposite no morethan one notch. This avoids the notches disturbing the air flow toomuch.

Rotary air heaters comprising such an undulating heating surface elementminimize the skew air flow.

According to an embodiment of the invention there is provided a stack ofheating surface elements with a primary direction, said stack comprisinga first heating surface element having a herringbone structure, saidherringbone structure having a plurality of regions, said plurality ofregions being arranged such that the boundary of regions is along saidprimary direction, said plurality of regions comprising a first regionhaving a plurality of undulations arranged laterally side by side, thelongitudinal extent of said undulations in said first region beinggreater than 0° and more than 90° to said primary direction, saidplurality of regions further comprising a second region, adjacent tosaid first region, said second region having a plurality of undulationsarranged laterally side by side, the longitudinal extent of saidundulations in said second region being less than 0° and more than −90°to said primary direction, said stack further comprising a secondheating surface element, said second heating surface element comprisinga notch extending along said primary direction, said notch beingarranged such that it is not directly opposite the boundary between saidfirst and second region.

Arranging the notch such that it is not directly opposite the boundarybetween the first and second regions allows the gas to flow along theherringbone structure and reduces the effect of vortices.

There are preferably a plurality of first regions each with a pluralityof undulations arranged laterally side by side, the longitudinal extentof the undulations being between +10° and +80° to the primary directionand a plurality of second region, each having a plurality of undulationsarranged laterally side by side, the longitudinal extent of theundulations being between −10° and −80° to the primary direction. Eachof the second regions is adjacent to at least one first region andpreferably the first and second regions alternate to create an overallherringbone structure.

There are preferably a plurality of notches extending along the primarydirection, at least one of the notches being arranged such that it isnot directly opposite a boundary between a first and second region inthe heating surface element directly opposing it. Indeed, preferablymore than one of the notches is not directly opposite a boundary betweenthe first and second region in a directly opposing heating surfaceelement.

Ideally, none of the notches directly oppose a boundary between a firstand second region in the heating surface element directly opposing thegas flow along the herringbone structure. To achieve this the secondheating surface element may be manufactured such that the notches arearranged at an equal spacing having a regular periodicity, and theherringbone structure also has a regular periodicity. To ensure that atleast some of the notches are not directly opposite the boundary betweenthe first and second regions the period, or distance between thelongitudinal notches is slightly greater than the period or distancebetween the boundaries between the first and second regions.

A stack or a pack of heating surface elements according to the inventionpreferably comprises a plurality of first and second heating surfaceelements arranged alternately. Such that a first heating surface elementis directly opposite to second heating surface element and a secondheating surface element is directly opposite to first heating surfaceelement.

The first and second regions of the first heating surface elementdirectly border each other, preferably such that the maximum of eachpeak of undulations in the first region substantially meets the maximumof a peak of undulations in the second region.

The angle of undulations is preferably between +15° and 35° in the firstregions and −15° to −35° to the primary direction in the second region.

The second heating surface element may additionally comprise undulationsarranged laterally side by side between the notches. The longitudinalextent of the undulations being either between +10° and +80° to theprimary direction or between −10° to −80° to the primary direction. Thisalso helps to direct the gas such that efficient heat transfer can beachieved.

The invention will now be described with reference to the accompanyingnon-limiting figures:

FIG. 1 depicts a undulating heating surface element according to theprior art;

FIG. 2 depicts a pack of undulating heating surface elements accordingto the prior art;

FIG. 3 depicts a undulating heating surface element according to theprior art;

FIG. 4 depicts a undulating heating surface element according to theprior art;

FIG. 5 is a cross section of a pack of heating surface elements of thecorrugated undulated type according to the prior art;

FIG. 6 is an alternative view of the pack of heating surface elementsshown in FIG. 5;

FIG. 7 shows a pack of heating surface elements disclosed in U.S. Pat.No. 6,019,160;

FIG. 8 depicts a pack of undulating heating surface elements accordingto the prior art;

FIG. 9 shows a pack of heating surface elements disclosed in U.S. Pat.No. 5,836,379;

FIG. 10 shows a pack of heating surface elements disclosed in U.S. Pat.No. 6,179,276;

FIG. 11 shows a heating surface element according to the invention;

FIG. 12 shows a pack of heating surface elements according to anembodiment of the invention;

FIG. 13 shows a schematic heating surface element according to theinvention;

FIG. 14 shows flow patterns along heating surface elements according tothe invention;

FIG. 15 shows flow patterns along heating surface elements according tothe invention; and

FIG. 16 depicts a pack of heating surface elements according to theprior art.

Having described the prior art, the subject of this invention isillustrated in FIGS. 11 to 14. This invention is more closely related tothe “double-undulated” and “corrugated-undulated” element profiles shownin FIGS. 1-6, than the more complicated double notch profiles such asthose shown in FIGS. 7, 9 and 10. Moreover, the invention provides amethod of dramatically reducing the flow dissipation and skew flowcharacteristics of these profiles by the simple expedient of modifyingthe geometry of only one sheet in the element pair—the undulated sheet.

FIG. 11 is a fragmentary view of the modified undulated sheet asproposed in this invention, while FIG. 12 is a fragmentary view of thissheet combined with a notched-undulated plate used in standarddouble-undulated elements. As can be seen in FIG. 11 the undulated sheetcomprises a first region of undulation in a first direction and a secondregion of undulations in a second direction with the peak of theundulations in the first region meeting the peak of the undulations inthe second region at a boundary. As can be seen in FIG. 12 the notchesin the second plate are not directly opposite the boundary between thefirst and second region of the first sheet. FIG. 13 shows a larger viewof the arrangement of the two sheets shown in FIG. 12 with the notchesbeing indicated by chain dotted lines, 6 and the herringbone shapedundulations 5 shown underneath. As can be seen most of the notches arenot superimposed at the apexes of the herringbone structure. In thisembodiment this is achieved by the notches having a slightly largerperiod between them than the period of the undulations. However, thiscould be achieved by any number of different methods.

The effect of arranging the notches such that they are not directlyopposite the boundary between the different directions of undulation canbe seen in FIGS. 14 and 15. The gas flows along the herringbonestructure and when the gas flow reaches the apex of the herringbonestructure it meets gas flowing in the opposite direction and istherefore directed back across the herringbone structure as shown inparticular in FIG. 14. As can be seen, the gas flow passes underneaththe notches such that the notches do not present significantinterference to the flow pattern and the gas and gas flow is notcompartmentalised by the notches. This results in an even flow at theoutlet. In contrast, if the notches were arranged to be opposite theapexes of the herringbone structure the combined effect of the notchesand the apex of the herringbone structure would aid the generation ofvortices between adjacent portions of undulations with opposingdirections.

This invention is not limited to use of the undulated sheet shown inFIG. 11 with the notched undulated plate shown in FIG. 12 but can beused in conjunction with other corrugated forms of sheets such as thoseshown in FIGS. 5 and 6. However crucially, at least one, and preferablymore of the notches should not be directly opposite the boundary betweenthe different directions of undulations.

FIG. 13 shows a typical wider view of a typical arrangement of thesezig-zag undulations across the larger area of the full element sheet.Note that the zig-zag undulations, 5, in this sheet are arranged in atransverse orientation across the sheet. Moreover, the angle (A) ofthese undulations is typically in the range 15°-35° to the direction offlow.

Note also on FIG. 13 that the typical positions of the opposing notchesin the adjacent sheets in the pair have been shown in chain-dottedvertical lines, 6. Moreover, close examination of the figure will showthat the size and angle of the zig-zags have been selected to ensurethat each undulation flowing in one direction crosses no more than oneopposing notch before the undulation changes direction. While this is anoptimum arrangement, it is not an essential part of the invention andthe zig-zags might be made larger in dimensions to cross two or morenotches or corrugations on the adjacent sheet.

FIGS. 14 and 15 shows the simplified, 2-dimensional internal flowpatterns that will occur within the element pack. This figure clearlyshows the purpose of the transverse zig-zags in the undulated sheet.Note that, on entry to the element pack the incoming flue gas or airwill tend to be deflected by the points of the V's in the undulations inone or the other direction across the plate, depending on the localdirection of the undulations. The result is that there will not be asingle consistent direction in which skew flow is allowed to develop asit passes through the element.

Moreover, as can be seen from the indicated flow pattern further downthe element, adjacent, slight skew flow streams will tend to convergetogether as the flow approaches the valleys of the V's. However, thistendency to converge will be resisted by the opposing components of theflow momentum such that the local flow direction will tend to bestraightened out or even reversed as shown in the lower part of theelement. The net effect of the transverse zig-zags across the plate willbe to eliminate the skew flow that would otherwise occur with normalundulated sheets, thereby producing an even flow across the outlet tothe element pack.

This elimination of skew flow effects and constriction of the transversedissipation of flow thereby helps to overcome all four of the potentialproblems of skew flow mentioned in the earlier description.

1. A stack of heating surface elements with a primary direction, said stack comprising a first heating surface element having a herringbone structure, said herringbone structure having a plurality of regions, said plurality of regions being arranged such that the boundary of regions is along said primary direction, said plurality of regions comprising a first region having a plurality of undulations arranged laterally side by side, the longitudinal extent of said undulations in said first region being greater than 0° and less than 90° to said primary direction, said plurality of regions further comprising a second region, adjacent to said first region, said second region having a plurality of undulations arranged laterally side by side, the longitudinal extent of said undulations in said second region being less than 0° and more than −90° to said primary direction, said stack further comprising a second heating surface element, said second heating surface element comprising a notch extending along said primary direction, said notch being arranged such that it is not directly opposite the boundary between said first and second region.
 2. A stack according to claim 1, wherein said first heating surface element comprises a plurality of first regions, each having a plurality of undulations arranged laterally side by side, the longitudinal extent of said undulations being between +10° and +80° to said primary direction, said first heating surface further comprising a plurality of second regions, each having a plurality of undulations arranged laterally side by side, the longitudinal extent of said undulations being between −10° and −80° to said primary direction, each of said second regions being adjacent to at least one first region along a boundary arranged along said primary direction.
 3. A stack according to claim 1, wherein said second heating surface element comprises a plurality of notches extending along said primary direction, at least some of said plurality of notches being arranged such that they are not directly opposite a boundary between a first region and a second region on said first heating surface element.
 4. A stack according to claim 1, wherein said stack comprises a plurality of first heating surface elements and a plurality of second heating surface elements, each first heating surface element being adjacent to at least one of said second heating elements.
 5. A stack according to claim 1, wherein said first region directly borders said second region.
 6. A stack according to claim 5, wherein, at the boundary between said first and second region, the maximum of each peak of undulations in said first region substantially meets the maximum of a peak of an undulation in said second region.
 7. A stack according to claim 1, wherein the longitudinal extent of undulations in said first region are at an angle of between +15° to +35° to said primary direction and the longitudinal extent of the undulations in said second region are at an angle of between −15° to −35° to said primary direction.
 8. A stack according to claim 1, wherein said second heating surface element comprises a plurality of undulations arranged laterally side by side, the longitudinal extent of said undulations being either greater than 0° and less than 90° to said primary direction or being less than 0° and more than −90° to said primary direction.
 9. A stack according to claim 1, wherein the distance between the notches in the second heating surface element is greater than the distance between the boundary between the first and second regions in the first heating surface element.
 10. A rotary air heater comprising a stack of heating surface element according to claim
 1. 